General purpose polyester resins can be used for most types of molding and laminating. By polyester is meant the polycondensation product of dicarboxylic acids with dihydroxy alcohols in contradistinction to materials known as alkyds. These polyester compounds may be modified by mono-carboxylic acids, monohydroxy alcohols and small amounts of polycarboxylic acids or polyhydroxy alcohols. Such compounds have a wide range of properties and a particular resin may have properties that may make it suitable for one purpose but unsuitable for another. For example, high viscosity resins are useful in vertical layup, where low viscosity resins, however, would be required when rapid penetration was desirable.
The wide range of properties possible with polyester resins leads to a variety of applications. They can be used as the primary polymer in fiber reinforced laminates and as the binder in composites containing a variety of inert fillers. Castings, potting compounds, cements, sealing and patching compounds, rigid and flexible coatings and adhesives can be based on polyester resins. As distinct from saturated polyesters, the resins contain olefins which produce the highly cross-linked structure generally known as a thermoset polymer.
While the uses of general purpose polyester resins continue to steadily grow, such resins suffer from the drawback that due to their high hydrogen and carbon content they continue to burn fairly readily once ignited. Since much concern has been generated by consumers to reduce the flammability of products such as flammable fabrics, manufacturers have had to find ways to reduce the flammability of polyesters and polyester resins.
The flammability of polyester resins can be reduced in several ways. These include chemical modification of one or more of the resin components, addition of organic fire retardants or addition of inorganic fillers and fire retardants.
A common chemical modification is replacing the diacid with a diacid containing halogen. Tetrachlorophthalic and tetrabromophthalic anhydrides are commonly used. One of the most widely used anhydrides is chlorendic anhydride, made by the Diels-Alder addition of hexachlorocyclopentadiene to maleic acid.
Other common polymer modifications include post bromination of the resin and the use of brominated diols. Halogenated styrene has been used and there are numerous known examples of the attachment of phosphorus-containing moieties to the diols, diacids or cross-linking olefins.
Two main types of organic compounds are commonly used as soluble, non-reactive additives. Halogen-containing aliphatic or aromatic compounds are commonly cited. Phosphorus compounds such as triethylphosphate are also well known in the art. Currently the combination of the two, i.e., tris-.beta.-chloroethylphosphate or 2,3-dibromopropyl phosphate have been widely used. In all of these cases the amount of additive is limited by possible plasticizing effects. Even small amounts of organometallics, examples being ferocene or ferocene derivatives have been recommended as desirable additives.
A number of inorganic additives are commonly added. Of course, glass fibers or cloth are used to increase the strength of the resin, but this may or may not help the flammability. The use of inert fillers such as calcium carbonate, magnesium oxide, etc. usually help the flammability characteristics in a minor way. The addition of hydrated salts and oxides, aluminatrihydrate being the most widely used, are effective by acting as a heat sink that slows down the energy transfer to the polymer. Antimony oxide is used, usually in conjunction with halogens, the latter may be incorporated as part of the resin or as a separate additive. Another commonly used inorganic additive is zinc borate.
Thus, while the techniques used to reduce the flammability of polyester resins do so to some extent, all suffer from some drawback.